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Abstract
Physical gelling of ionic liquid (IL) using polymers is a facile and efficient approach to stabilize IL for carbon dioxide (CO2) separation and to fabricate thin films with high flux. Despite the process simplicity, most of the investigated gelled membranes were fabricated as thick, free-standing membranes and thus impractical for real applications. In this work, 1-ethyl-3-methylimidizaolium tetrafluoroborate [emim][BF4] IL was gelled using a commercial block copolymer (Pebax®1657) by dissolving them in environmentally benign alcohol-based solvents. The intrinsic gas separation properties of the gelled dense membranes were first determined. The preferential interaction between IL and poly(ethylene oxide) segments via hydrogen bonding was found to decrease the polymer crystallinity, resulting in a larger area available for gas transport. Consequently, up to 80wt% IL could be gelled to achieve 300% increase in CO2 permeability compared to that of neat Pebax membranes.
To enhance the gas separation properties, Graphene Oxide (GO) nanofillers were incorporated to form mixed-matrix membranes. While GO lost its molecular sieving property, its alignment in the polymer/IL matrix and association with IL via π-π or cation-π interactions enhanced the surface concentration of IL and film polarity. The CO2 sorption and permeation were subsequently improved, particularly for the films fabricated in alkaline condition with CO2 permeance of 1000 GPU and CO2/N2 selectivity of 44. In a separate study, the effects of common inorganic salts on the morphology and performance of Pebax-IL gel membranes were examined. Highly hygroscopic salts such as CaCl2 were found to increase the amount of membrane water uptake, which also led to improved CO2 permeation due to increase in gas diffusivity associated with water vapor-induced plasticization. Both blending strategies confirm the validity of solution-diffusion mechanism through the gelled IL membranes as well as the prevailing permeability-selectivity trade-off.
Furthermore, preparation of defect-free thin film composite (TFC) of Pebax/IL membranes and their derivatives the aforementioned membranes using dip-coating technique was shown to improve the mechanical stability to meet the industrial requirement. In addition, examinations on the effects of various operating conditions on the TFC gel membranes showed excellent CO2 separation performance and chemical stability, even with mixed-gas feed containing traces of contaminants.